1. Field of the Invention
The present invention relates generally to a thin film transistor substrate and a manufacturing method thereof. Particularly, the present invention relates to a polycrystalline silicon thin film transistor substrate in which two types of thin film transistors with differing characteristics are implemented on one substrate.
2. Description of the Related Art
A liquid crystal display apparatus is relatively light weight and thin, and has a low power consumption rate, and is therefore widely used in such fields as mobile terminal apparatuses, video camera view finders, and laptop computers, for example. In recent years and continuing, in an attempt to reduce costs, a polycrystalline silicon thin film transistor (poly-Si TFT) substrate that implements thin film transistors for driving pixels of a display area and thin film transistors for peripheral circuits outside the display area is being used. Particularly, a low temperature polycrystalline silicon thin film transistor substrate implementing a glass substrate is low-priced and may be easily enlarged. Thereby, such substrates are used not only in liquid crystal display apparatuses but organic EL apparatuses as well.
FIG. 1 is a diagram showing a configuration of a polycrystalline silicon thin film transistor substrate according to the conventional art. The polycrystalline silicon thin film transistor substrate of FIG. 1 includes a glass substrate 180 on which a pixel region 183 is implemented in the form of a matrix, pixel transistors 182 that are formed on portions of the pixel region 183, and periphery circuits 181. In order to form the pixel transistors 182 and the thin film transistors of the periphery circuits 181, a polycrystalline silicon film needs to be formed on the glass substrate 180. In the case of using a quartz substrate, which is high-priced, polycrystalline silicon may be directly laminated on the substrate by means of a high temperature thermal process; however, in the case of using a glass substrate, which is lower in price, a low-temperature process is implemented.
Accordingly, in many cases, a low temperature polycrystalline silicon thin film transistor is manufactured by initially forming an amorphous silicon (a-Si) film, and then irradiating an excimer laser on this film to form a polycrystalline film. The average crystal grain diameter of the polycrystalline silicon film that is crystallized in this manner depends on the power of the excimer laser. Specifically, the grain diameter increases with the increase of the laser power. When the average grain diameter is increased, the mobility of the polycrystalline silicon thin film transistor also increases. However, after reaching a predetermined threshold value, micro-crystallization occurs, and as a result, an inconsistency occurs in the mobility of the polycrystalline silicon thin film transistor.
Also, with the same laser power, the average grain diameter of the polycrystalline silicon film crystallized from an amorphous state tends to decrease as the film thickness of the original amorphous silicon film increases. Taking this factor into consideration, for example, in Japanese Patent Laid-Open Publication No. 11-284188, a technique of arranging the active layers of the transistors of the peripheral circuits, which require high speed operation, to be thinner than the active layers of the pixel transistors is proposed so as to increase mobility of the polycrystalline silicon thin film transistor. With the excimer laser that is presently used, when a film thickness of the amorphous silicon (a-Si) is arranged to be 60 nm or less, an n channel TFT with a TFT mobility of approximately 100 cm2/Vs may be realized.
In Japanese Patent Laid-Open Publication No. 6-125084, a polycrystalline silicon thin film transistor manufacturing method is disclosed, the method including forming a pixel transistor with a thin semiconductor layer, forming a peripheral transistor requiring high speed operation with a thick semiconductor layer, and crystallizing the semiconductor layers through a thermal annealing process.
Also, presently, a lateral crystallization method using CW (continuous wave) laser is attracting much attention as a crystallization method for realizing a TFT with higher mobility. In lateral crystallization, the crystal grain diameter increases along the scanning direction (lateral direction) of the laser. By forming the source/drain regions of the TFT along the crystallization direction extending laterally, an even higher mobility may be realized.
The crystallization method using CW laser has been contemplated in the conventional art, but the output laser power of the conventional CW laser is inconsistent and thereby uniform crystallization cannot be realized. However, recently, a technique of fixing the CW laser through laser diode excitation has been developed, and with this technique, the problem of inconsistency in the output laser power has been greatly reduced, thereby realizing a suitable crystallization method through CW laser irradiation.
With the CW laser irradiation method, the laser beam is arranged to have an oval-shaped laser spot and its spot diameter is reduced to several dozen μm in the minor axis direction, and several hundred μm in the major axis direction. The CW laser is arranged to be scanned at a speed ranging from several dozen to several hundred cm/s. In this way, a crystal grain diameter that cannot be obtained by using the excimer laser may be obtained using the CW laser. In the CW laser irradiation method, the laser beam absorption rate increases as the film thickness of the amorphous silicon (a-Si) film increases. Thereby, a large grain diameter may be achieved in the crystallization with low power. For example, when the film thickness of the amorphous silicon (a-Si) film is 50 nm or more, an n channel TFT with a mobility of 300 cm/Vs or higher may be realized.
However, in the polycrystalline silicon thin film transistor (poly-Si TFT) manufactured through lateral crystallization using CW laser, the following problems become prominent.
(1) Since the film thickness of the amorphous silicon (a-Si) film is increased compared to the case of using an excimer laser, leak current is increased during the off time of the transistor.
(2) With a CW laser beam, which has an oval-shaped beam spot, as opposed to the excimer laser, which has a narrow slit-shaped beam configuration, more time is required in laterally crystallizing the entire surface of the substrate thereby decreasing productivity. In this aspect, a technique of using plural beams may be contemplated, but in such case, it is difficult to maintain consistency in the beam energy, and the yield may be decreased.
(3) Since a high TFT mobility is achieved, the pressure resistance between the source/drain is degraded. In turn, the gate insulating film may be made thinner to reduce operation pressure; however, in such case, the pressure resistance with respect to gate voltage may be degraded.